Various cellular radio systems enabling personal wireless data transmission have been in use for long. An example of the mobile communication systems is the European digital mobile communication system GSM (Global System for Mobile Communications). In addition, a new generation of radio systems is being developed, which is expected to further expand the possibilities of personal wireless communication. One of these systems under standardization is the Personal Communication System (PCS) in the United States. In accordance with the present expectations, there will be two types of PCS networks, namely traditional overlay type networks, and fixed ISDN and PSTN networks. There will be a number of alternatives for the radio interface, one of which is the GSM based PCS1900 (a GSM system operating within the frequency range of 1900 MHz).
Modification of an existing mobile communication standard (such as GSM or DCS1800) to a PCS-overlay network is a relatively straightforward process. Instead, more problems will be caused by PCS networks similar to the fixed ISDN and PSTN networks, employing a standard local exchange (LE) of the fixed network instead of a mobile services switching centre, such as an ISDN/PSTN based network is not directly compatible with the GSM network architecture. The mobile services switching centre is the central entity of the network in a GSM based overlay network. In an ISDN/PSTN based network, however, the local exchange LE does not have any functions supporting the mobile communication network. In the future, the local exchange LE may be assumed to include a small set of general functions that support switching of mobile communication networks (non-call related signalling, handover switching support of some kind), but they are not necessarily very useful for meeting the requirements of the PCS1900 system, for instance.
A solution to this problem is disclosed in "Switching and signalling generic requirements for network access services to personal communication services (PCS) provider", GR-2801-CORE, Issue 1, December 1993, Bellcore, Bell Communications Research, particularly chapter 1, pp. 1 to 10, chapter 2, pp. 1 to 26, and chapter 3, pp. 133 to 137. In a Bellcore network disclosed in the above document, the radio interface is a wireless access communication system WACS (Bellcore TR INS-001313), which is connected to the local exchange of a fixed network. An intelligent network is connected to the local exchange, providing the local exchange with the service logic, the databases and the operations required for the mobility management of the access network. In other words, an AIN network (Advanced Intelligent Network) provides the subscriber database services, location management, authentication, access control etc. in an integrated manner. The basic features of call control are located in an ISDN local exchange, from which call control can initiate intelligent network operations in accordance with predetermined triggers via a standard AIN interface. The Bellcore system also supports handover, which is referred by Bellcore to as "Automatic Link Transfer" (ALT). The aim of ALT is to ensure the continuity of a service to the user when the signal strength varies during an answered call. The signal strength may vary e.g. if a PCS user is moving, or if the conditions of the radio environment change. The aim of ALT is to enable the continuity of an answered ongoing call without interruption, and without the user taking any measures for performing an ALT or being aware of the fact the ALT has taken place. Chapter 3, pp. 133 to 138 of the above-mentioned document discloses an ALT procedure in which a local exchange operates as the "anchor point" of handover. In other words, when an ALT is carried out from an old base station system (IPS/RPCU) to a new base station system (IPS/RPCU), the connection to the old base station system is released in the exchange, and a new connection is established to the new base station system. In such a case, the local exchange for the fixed network is the anchor point of switching and signalling. The handover employing the local exchange as the anchor point may be carried out in new national ISDN exchanges having the required features. This handover concept, however, cannot be considered as an appropriate way of proceeding, particularly at the initial stage of the PCS systems, because most of the existing exchanges of the fixed network represent the older type, and it is not possible to guarantee the availability of exchanges that have the required features. Furthermore, problems are caused by the fact that a network element incompatible with the access network standard, such as the GSM, is used as the anchor point of handover connections.
PCT Publication WO 95/01694 discloses a handover between base station systems under the local exchange of a fixed network whereby the anchor point for handover is maintained in the mobile communication system. In other words, the first base station controller in the call operates as the anchor point to which the initial connection established via the local exchange is maintained for the entire duration of the call. In the handover, a further connection is established from the anchor base station controller to a second base station controller.